1. Field of the Invention
The invention pertains to a watchdog unit for current or voltage supply system, in which a supply voltage is furnished via at least one safety cutoff switch to two or more outputs.
2. Description of the Related Art
In industrial systems and in automated systems, e.g., a building's automation system, it is especially important that the supply voltages be furnished to the individual consumers or groups of consumers according to special safety criteria. In particular, information-processing parts of a control system, such as microprocessor components, must be supplied with the necessary energy as a priority in event of malfunction. In many industrial applications, the supply voltage is 24 volts dc voltage, although other dc voltage values are also customary and alternating voltages such as 115, 230 or 24 volts are also in use.
In event of short-circuit or overload in certain areas of a control system, such as the output peripherals, there can easily occur what is often only a brief total outage of the supply voltage of the control system, which often results in a loss of data in the central control units receiving the supply voltage.
When supply voltages are generated by main transformers connected to rectifiers, the safety cutoff switches used are line protection switches or as is also customary, melting fuses. The ability of transformers to briefly furnish a multiple of their rated current makes it possible to cut out a circuit with a short-circuit or an overload without triggering the safety features at the primary side. Although a transient voltage dip will occur, especially during short circuit, whose amplitude will depend on the stray inductance of the transformer and on the line impedances, proper dimensioning in most instances can prevent a so-called “crash” of central control units.
The increasing use of switching-mode power supplies, i.e., clocked power supply systems, makes this situation once again urgent, because of the sensitive electronics the internal control circuits limit the output current to values which lie only slightly above the rated current. In particular, the problem arises that the safeguards cannot shut off in sufficiently short time for the individual outputs. For quick shut-off, safeguards or power protection switches often require a multiple of their rated current. However, the switching-mode power supplies cannot furnish this in addition to the usual load, so that the entire supply voltage drops—even before a safeguard is triggered and the faulty output or branch is cut off. Such voltage dips associated with a hard-to-estimate magnitude can then lead to the already-mentioned crashing of central control units. If one wishes to preclude or greatly limit this danger, the only way to do this is to provide totally separate current supply devices for each individual circuit, such as peripheral subassemblies, on the one hand, and a CPU subassembly, on the other. But of course this solution is extremely costly in terms of expense and the required space and weight.